In water pumps of the above type, sealing between the centrifugal impeller (and relative shaft) and the casing is effected using a sealing assembly comprising a first sealing ring for sliding sealing engagement fitted to the pump casing, coaxially with the impeller shaft, and which also exert a static sealing on the casing; and a second sealing ring for sliding sealing engagement fitted to the impeller shaft, which as well exerts a static sealing on the shaft, and which is loaded axially by a spring packed between the rear face of the impeller (opposite the front work face of the impeller fitted with the blades) and the second ring. Dynamic in-service sealing is therefore performed by the sliding seal defined between the two rings, which are pressed into contact with each other frontally by the spring in a direction parallel to the axis of rotation (axial sealing).
Given the high axial loads the seal is normally subjected to in use, sliding sealing rings are normally made of metal. As a result, sealing of the water in the delivery compartment of the pump is not altogether satisfactory, especially when the pump is idle. Moreover, the known sealing assembly described is bulky, particularly axially, and is complicated to assemble.